Field of the Invention
Embodiments of the present disclosure relate generally to energy storage in a microgrid and, more particularly, to operation of AC-coupled energy storage devices in a microgrid.
Description of the Related Art
A conventional microgrid generally comprises at least one energy generator, at least one energy storage device, and at least one energy load. When disconnected from a conventional utility grid, a microgrid can generate power as an intentional island without imposing safety risks on any line workers that may be working on the utility grid.
Droop control is one technique that may be used for operating energy storage and generation resources in a microgrid that is disconnected from the utility grid. Droop control is an industry standard technique for autonomously sharing load among parallel AC generators proportional to their power ratings or operating costs. The technique relies on using small changes in voltage and frequency to dictate changes in real and reactive power levels. The “phase shift virtual impedance droop control” method is a time-domain implementation of droop control by which the converter is controlled to appear as a virtual AC voltage source in series with a virtual impedance, where the virtual AC voltage source has a constant amplitude and is phase-shifted proportional to the error between the measured grid frequency and the nominal grid frequency.
This technique has several advantages including improved dynamic response and harmonic compensation; however, the method loses direct control of real and reactive currents and thus makes it difficult to impose current limits. Current limits are necessary to constrain the converter to a safe or desired operating region. For example, if the virtual source voltage phasor commanded during droop control would result in a real or reactive current phasor that exceeds the maximum capability of the converter, the converter could be damaged or be forced to shut-down. In addition, by not having direct phasor control of the real and reactive currents, a converter cannot be operated with a virtual impedance while grid connected, resulting in disjointed transitions between islanded and grid-connected states.
When sufficient energy is available during microgrid operation, energy storage resources (such as batteries) can be charged based on their droop characteristics. However, the maximum rate of charging for the energy storage resources will be reduced when the energy storage resources are providing reactive power for one or more loads, thereby reducing the economic value.
Therefore, there is a need in the art for an efficient technique for controlling reactive power in AC coupled batteries while ensuring energy harvest.